FIG. 6 shows an example of a rolling bearing unit for supporting a wheel disclosed in Patent Literature 1. A rolling bearing unit 1 for supporting a wheel shown in FIG. 6 includes an outer ring 2, a hub 3, and a plurality of rolling elements (in the example shown, balls) 4 and 4.
The outer ring 2 has a stationary side flange 5 formed on an outer circumferential surface thereof, and a plurality of rows of outer ring tracks 6a and 6b formed on an inner circumferential surface thereof. In addition, the hub 3 is formed by assembling a hub main body 7 and an inner ring 8. The hub main body 7 has a rotary side flange 9 formed on a portion of the hub main body 7 close to an outer end of an outer circumferential surface in an axial direction, an inner ring track 10a that is formed on an intermediate section of the hub main body 7 in the axial direction and that is disposed outside of the intermediate section in the axial direction, and a small diameter step section 11 formed on an inner end portion of the hub main body 7 in the axial direction. Further, “an axial direction,” “a radial direction” and “a circumferential direction” in the specification and claims are directions with respect to a hub main body unless the context clearly indicates otherwise. In addition, “an outer side” in the axial direction in the specification and claims represents an outer side in a widthwise direction of the vehicle when assembled to an automobile, that is, the left side of FIG. 6. On the other hand, the right side of FIG. 6 that is a central side of the vehicle when assembled to the automobile is referred to as “an inner side” with respect to the axial direction.
The inner ring 8 has an inner ring track 10b that is formed on an outer circumferential surface thereof and formed inside in the axial direction, and the inner ring 8 is fastened and fitted onto the small diameter step section 11 of the hub main body 7. In addition, the plurality of rolling elements 4 and 4 is installed between the outer ring tracks 6a and 6b and the inner ring tracks 10a and 10b for each row in a rollable manner. In addition, in this state, a caulking section 13 is formed by plastically deforming a portion of a cylindrical section 12, formed on an inner end portion of the hub main body 7 in the axial direction, protruding from an inner end opening of the inner ring 8 in the axial direction outward in the radial direction. Then, an appropriate preload is applied to the rolling elements 4 and 4 as an inner end surface of the inner ring 8 in the axial direction is pressed by the caulking section 13.
When the rolling bearing unit 1 for supporting a wheel configured as described above is assembled, first, the outer ring 2 is disposed around the hub main body 7, and the rolling elements 4 and 4 are installed between the outer ring track 6a outside in the axial direction among the outer ring tracks 6a and 6b and the inner ring track 10a outside in the axial direction while being held by a retainer 14a outside in the axial direction. Next, the rolling elements 4 and 4 are installed around the inner ring track 10b inside in the axial direction formed on the outer circumferential surface of the inner ring 8 while being held by a retainer 14b inside in the axial direction, and in this state, the inner ring 8 is fastened and fitted onto the small diameter step section 11 formed on the inner end portion of the hub main body 7 in the axial direction. Then, according to the on-fitting work, rolling surfaces of the rolling elements 4 and 4 held by the retainer 14b inside in the axial direction abut the outer ring track 6a outside in the axial direction. Next, the caulking section 13 is formed by plastically deforming an inner end portion of the cylindrical section 12 of the hub main body 7 in the axial direction (a portion of the inner ring 8 protruding from an inner end opening in the axial direction) outward in the radial direction. Then, the inner ring 8 can be fixed to the hub main body 7 as the inner end surface of the inner ring 8 in the axial direction is suppressed in the axial direction by the caulking section 13.
The caulking section 13 configured to fix the inner ring 8 to the hub main body 7 is formed by, for example, a rotating and caulking process as disclosed in Patent Literatures 1 to 3. When the caulking section 13 is formed by the rotating and caulking process, for example, a rotating type caulking apparatus 15 as shown in FIG. 7 is used. The rotating type caulking apparatus 15 includes a mold 16, a pressing tool 17 and a holder 18. When the caulking section 13 is formed by caulking and spreading the cylindrical section 12, the mold 16 is rotated and rotated by an actuator (not shown) while the hub 3 is pressed upward via the holder 18. That is, in a state in which a central axis of the mold 16 and a central axis of the hub 3 are inclined by an angle θ, the mold 16 is rotated about the central axis of the hub 3. When the caulking section 13 is formed by the rotating and caulking process performed using the above-mentioned rotating type caulking apparatus 15, machining work is partially and continuously performed on the caulking section 13 in the circumferential direction (the central axis of the mold 16 is rotated along a trajectory of a central axis due to a precessional motion around the central axis of the hub 3) since a portion of the mold 16 in the circumferential direction presses an inner end portion of the cylindrical section 12 in the axial direction. For this reason, in comparison with the case in which the caulking section 13 is formed by general forging, a load required for machining can be decreased.
The caulking section 13 formed as described above applies an appropriate preload to the rolling elements 4 and 4 by pressing the inner end surface of the inner ring 8 in the axial direction. In order to apply a preload having an appropriate magnitude as described above, it is desirable that the caulking section 13 makes a force pressing the inner end surface of the inner ring 8 in the axial direction outward in the axial direction (an axial force) constant (stabilize it). For this reason, in the case of the rotating and caulking process is performed using the above-mentioned rotating type caulking apparatus 15, a load applied to the cylindrical section 12 by the mold 16 and the time for applying such a load are kept constant. However, in the case of such a method, a magnitude of the applied load and the time for applying the load may not be optimized for every workpiece (according to a difference in manufacturing error or property). That is, for example, depending on a variation or the like of a protrusion amount (a protrusion length) of a portion of the cylindrical section 12 protruding from the inner end opening of the inner ring 8 in the axial direction based on the manufacturing error, there is a possibility that a load larger than a load required for applying a preload having a desired magnitude needs to be applied or the time for applying the load needs to be excessively increased.